DEV: Clean up .gitignore (#12981)

(inspired by martin-brennan's recent adventure with `tags` exclusion)

**tl;dr: some of it is obsolete, some is dev-env specific and should live in *your* ~/.gitignore.**

---

To enable global gitignore (`~/.gitignore`) run:
```bash
git config --global core.excludesFile '~/.gitignore'
```

Then create that file and add your VIM/VSCode/Sublime/Emacs/Eclipse/RubyMine/JetBrains/macOS/Arch/Windows 95-specific stuff.

---

Reasons for removal:

* `bin` - never really ignored, all the files are checked in
* `.DS_Store`, `._.DS_Store` - OS specific
* `.sass-cache/*`, `/.bundle`, `/bundle/*`, `/cache`, `/logfile`, `!/plugins/discourse-nginx-performance-report`, `MiniProfiler/Ruby/rack-mini-profiler-2.0.1a.gem`, `config/fog_credentials.yml`, `/public/stylesheet-cache/*`, `script/download_db`, `script/refresh_db`, `config/version.rb` - no longer used?
* `/log/*.log` - covered by /log
* `bootsnap-load-path-cache`, `bootsnap-compile-cache` - bootsnap now keeps its ~~trash~~ temporary files in `/tmp`
* `/.project`, `/.buildpath`, `/.byebug_history`, `/.idea`, `discourse.sublime-workspace`, `*~`, `*.swp`, `*.swo`, `*.swm`, `config/multisite1.yml`, `.rvmrc`, `.ruby-version`, `.ruby-gemset`, `.rbenv`, `bundler_stubs/*`, `*.db`, `*.iml`, `*.swn`, `/package-lock.json`, `.vscode`, `.envrc`, `tags` - dev-env specific, see the note at the top of the commit

bin/rubocop

@@ -0,0 +1,29 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+#
+# This file was generated by Bundler.
+#
+# The application 'rubocop' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+require "pathname"
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
+  Pathname.new(__FILE__).realpath)
+
+bundle_binstub = File.expand_path("../bundle", __FILE__)
+
+if File.file?(bundle_binstub)
+  if File.read(bundle_binstub, 300) =~ /This file was generated by Bundler/
+    load(bundle_binstub)
+  else
+    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
+Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
+  end
+end
+
+require "rubygems"
+require "bundler/setup"
+
+load Gem.bin_path("rubocop", "rubocop")