FEATURE: Add support for case-sensitive Watched Words (#17445)

* FEATURE: Add case-sensitivity flag to watched_words

Currently, all watched words are matched case-insensitively. This flag
allows a watched word to be flagged for case-sensitive matching.
To allow allow for backwards compatibility the flag is set to false by
default.

* FEATURE: Support case-sensitive creation of Watched Words via API

Extend admin creation and upload of Watched Words to support case
sensitive flag. This lays the ground work for supporting
case-insensitive matching of Watched Words.

Support for an extra column has also been introduced for the Watched
Words upload CSV file. The new column structure is as follows:

 word,replacement,case_sentive

* FEATURE: Enable case-sensitive matching of Watched Words

WordWatcher's word_matcher_regexp now returns a list of regular
expressions instead of one case-insensitive regular expression.

With the ability to flag a Watched Word as case-sensitive, an action
can have words of both sensitivities.This makes the use of the global
Regexp::IGNORECASE flag added to all words problematic.

To get around platform limitations around the use of subexpression level
switches/flags, a list of regular expressions is returned instead, one for each
case sensitivity.

Word matching has also been updated to use this list of regular expressions
instead of one.

* FEATURE: Use case-sensitive regular expressions for Watched Words

Update Watched Words regular expressions matching and processing to handle
the extra metadata which comes along with the introduction of
case-sensitive Watched Words.

This allows case-sensitive Watched Words to matched as such.

* DEV: Simplify type casting of case-sensitive flag from uploads

Use builtin semantics instead of a custom method for converting
string case flags in uploaded Watched Words to boolean.

* UX: Add case-sensitivity details to Admin Watched Words UI

Update Watched Word form to include a toggle for case-sensitivity.
This also adds support for, case-sensitive testing and matching of  Watched Word
in the admin UI.

* DEV: Code improvements from review feedback

 - Extract watched word regex creation out to a utility function
 - Make JS array presence check more explicit and readable

* DEV: Extract Watched Word regex creation to utility function

Clean-up work from review feedback. Reduce code duplication.

* DEV: Rename word_matcher_regexp to word_matcher_regexp_list

Since a list is returned now instead of a single regular expression,
change `word_matcher_regexp` to `word_matcher_regexp_list` to better communicate
this change.

* DEV:  Incorporate WordWatcher updates from upstream

Resolve conflicts and ensure apply_to_text does not remove non-word characters in matches
that aren't at the beginning of the line.

spec/lib/topic_creator_spec.rb

@@ -104,6 +104,25 @@ RSpec.describe TopicCreator do
         end
       end
 
+      context 'when assigned via matched watched words' do
+        fab!(:word1) { Fabricate(:watched_word, action: WatchedWord.actions[:tag], replacement: tag1.name) }
+        fab!(:word2) { Fabricate(:watched_word, action: WatchedWord.actions[:tag], replacement: tag2.name) }
+        fab!(:word3) { Fabricate(:watched_word, action: WatchedWord.actions[:tag], replacement: tag3.name, case_sensitive: true) }
+
+        it 'adds watched words as tags' do
+          topic = TopicCreator.create(
+            user,
+            Guardian.new(user),
+            valid_attrs.merge(
+              title: "This is a #{word1.word} title",
+              raw: "#{word2.word.upcase} is not the same as #{word3.word.upcase}")
+          )
+
+          expect(topic).to be_valid
+          expect(topic.tags).to contain_exactly(tag1, tag2)
+        end
+      end
+
       context 'staff-only tags' do
         before do
           create_staff_only_tags(['alpha'])