Extract changelog template parser to new module

What does this MR do?

This MR extracts template parser from Gitlab::Changelog (added in !50063 (merged) and modified in !53239 (merged) ) to a new module, Gitlab::TemplateParser.

This template parser was added to allow users to write custom changelog templates. However, it could also be used for:

• issue templates ( #26764 )
• merge commit templates ( #20263 (closed) )

I was thinking of implementing merge commit templates. I saw @yorickpeterse comment here #26764 (comment 521326645) about moving changelog template parser to a new namespace, so I've decided to start with this.

Screenshots (strongly suggested)

This MR does not change anything visible to user, this is only internal refactor.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.