Alerting: Cleanup and move legacy to a legacy file (#32803)

* Alerting: Cleanup and move legacy to a legacy file

A quick cleanup of the ngalert/api directory, optimising for an easy
removal of what is will be considered legacy at some point. A quick
summary of what's done is:

- Add a prefix `generated` prefix to files that are auto-generated by
  our swagger definitions.
- Create a legacy file to place all the legacy API routes implementation
  and helpers. Deleting files that where no longer needed after this
move.
- Rename the `lotex` file to `lotex_ruler`
- Adding a couple of comments here and there.

With this, I hope to organise our code in this directory a bit better
given there's a lot going on.

go.sum

@@ -1041,6 +1041,7 @@ github.com/jung-kurt/gofpdf v1.0.3-0.20190309125859-24315acbbda5/go.mod h1:7Id9E
 github.com/jung-kurt/gofpdf v1.16.2 h1:jgbatWHfRlPYiK85qgevsZTHviWXKwB1TTiKdz5PtRc=
 github.com/jung-kurt/gofpdf v1.16.2/go.mod h1:1hl7y57EsiPAkLbOwzpzqgx1A30nQCk/YmFV8S2vmK0=
 github.com/jwilder/encoding v0.0.0-20170811194829-b4e1701a28ef/go.mod h1:Ct9fl0F6iIOGgxJ5npU/IUOhOhqlVrGjyIZc8/MagT0=
+github.com/k0kubun/colorstring v0.0.0-20150214042306-9440f1994b88 h1:uC1QfSlInpQF+M0ao65imhwqKnz3Q2z/d8PWZRMQvDM=
 github.com/k0kubun/colorstring v0.0.0-20150214042306-9440f1994b88/go.mod h1:3w7q1U84EfirKl04SVQ/s7nPm1ZPhiXd34z40TNz36k=
 github.com/k0kubun/go-ansi v0.0.0-20180517002512-3bf9e2903213/go.mod h1:vNUNkEQ1e29fT/6vq2aBdFsgNPmy8qMdSay1npru+Sw=
 github.com/kardianos/osext v0.0.0-20190222173326-2bc1f35cddc0/go.mod h1:1NbS8ALrpOvjt0rHPNLyCIeMtbizbir8U//inJ+zuB8=