Cache issuable template names

What does this MR do?

First commit: don't use the basename of a file when detecting if it's a special file, but the full path.

Second commit: add this caching of special repository contents to issue and MR template names.

Are there points in the code the reviewer needs to double check?

Is the first commit valid? I think it is.

Why was this MR needed?

We spend a surprising amount of time when loading an issue page getting these names:

It also stops one source of repository access on issue load.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/27164.