[Proposal] Make Gitea template system support context function

[wxiaoguang]

Plan

After #24056, we have our own ScopedTemplate, which supports context functions.

This is a plan for next steps:

• Wait for enough time to test and verify that our ScopedTemplate is stable enough
  • Maybe in about one month, use the edge build on gitea.com to eat our own dogfood
  • If there is anything wrong:
    • If it is fixable, improve the ScopedTemplate
    • If it is unfixable, revert to Golang's html/template (pity.....)
• If Golang supports context function officially soon, switch to the official solution.

Then, if everything is fine:

• Make locale use context function as the first refactoring step (maybe it, or maybe some other simple functions)
• (TBC)

[wxiaoguang]

Wait for Golang's official solution

https://go-review.googlesource.com/c/go/+/510738