Adding Support For Searching Tags Using `^` and `$`.

What does this MR do?

This adds in support for using start/end regex-like markers (^ and $) when searching through tags. This matches the branches searching feature.

This change also makes slight cleanups to branches tests / documentation (adding test cases, grouping them in more appropriate location).

Screenshots

N/A

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

There should be no impact to performance. This uses (some of) the same logic -- although duplicated over -- that is already used for branch searching.

Added test cases to spec\finders\tags_finder_spec.rb to ensure correct functionality.

As this is a back-end change, cross-browser testing should not be necessary.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

This change does not add new params (and thus does not change the usage of defined()/defined_params(). It is using the pre-existing search param for the tags API. This change is only adding on functionality to the search param.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team