[Proposal] Clean up and refactor the template helper functions

[wxiaoguang]

Feature Description

Gitea is heavily using Go HTML Template, so it's worth to make the template system more maintainable.

There are some strange smells in template/helper.go:

• Deadcode clean up
  • Clean template/helper.go #23922
• Dict / dict / mergeinto: actually we can have only one unique "dict"(map) maintaining function.
  • Merge template functions "dict/Dict/mergeinto" #23932
• Many calculation related functions like Add, Mul, percentage, etc, they could be replaced by a more generic Eval like: {{Eval "(" $a "+" $b ")" "/" 100}}
  • Use a general Eval function for expressions in templates. #23927
• Printf vs printf: only builtin printf is enough
  • Group template helper functions, remove Printf, improve template error messages #23982
• Iterate: it seems a little complex (removed by Improve Wiki TOC #24137)
• More and more functions need to use Context, it's better to introduce a general helper map or struct, then old code {{RenderEmoji $.Context ...}} could be improved to {{.CtxFunc.RenderEmoji ...}}
  • After Drop "unrolled/render" package #23965
  • We can have per-request context global functions.
  • Make HTML template functions support context #24056
  • [Proposal] Make Gitea template system support context function #24228
• containGeneric vs contain: clarify their behavior
  • Improve template helper functions: string/slice #24266

TODO:

• Use "SettingUI": &setting.UI to expose all UI settings to templates

ps: only do big refactoring after 1.19.0 or even 1.19.1 gets released, to prevent from making backport difficult.