Enforce max chars and max render time in markdown math

See merge request gitlab/gitlabhq!3277

https://gitlab.com/gitlab-org/gitlab-ce/issues/62971

Adds support for embedding specific charts from the
metrics dashboard. Expected parameters are dashboard,
title, group, and y_label.

KaTeX math will now render progressivly and asynchronously. There
are upper limits on the character count of each formula, and on
cumulative render time.

This adds support for a number of new languages (e.g. Ada)
and fixes a number of lexers:
https://github.com/rouge-ruby/rouge/compare/v3.5.1..v3.7.0

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64625

Add frozen_string_literal to spec/features

See merge request gitlab-org/gitlab-ce!31131

Using the sed script from
https://gitlab.com/gitlab-org/gitlab-ce/issues/59758

This allows using `CacheMarkdownField` for models that are not backed
by ActiveRecord.

When the including class inherits `ActiveRecord::Base` we include
`Gitlab::MarkdownCache::ActiveRecord::Extension`. This will cause the
markdown fields to be rendered and the generated HTML stored in a
`<field>_html` attribute on the record. We also store the version
used for generating the markdown.

All other classes that include this model will include the
`Gitlab::MarkdownCache::Redis::Extension`. This add the `<field>_html`
attributes to that model and will generate the html in them. The
generated HTML will be cached in redis under the key
`markdown_cache:<class>:<id>`. The class this included in must
therefore respond to `id`.