Refactor HTTP request context

[wxiaoguang]

Background

I was working on some new enhancements, while I found the modules/context/context.go is quite messy, so I think it's better to refactor first before other works.

Major Changes

• HTMLString(name string, data interface{}) => RenderToString(name base.TplName, data map[string]interface{}), the first paramter is a TplName, and the old HTMLString naming conflicts with other function names like HTML/PlainText
• TmplLoadTimes is reworked. The old TmplLoadTimes in old HTMLString has no effects because the render doesn't use ctx.Data
• NotFound(title string, err error) and ServerError(title string, err error) related functions are refactored, because the title is not title, it only appears in log.
• use PlainText(string) and PlainTextBytes([]byte) to replace old PlainText([]bytes) and HandleText(string)
• remove unused SignedUserName
• use golang standard type contextKeyType struct{}; var contextKey interface{} = contextKeyType{} to replace var contextKey interface{} = "default_context"
• use ctx.RespHeader() to replace ctx.Header(), because ctx.Header() is not clear whether it is for request or response.

[wxiaoguang]

CI failure is not related. Other passed PRs also have such failure:

        testlogger.go:78: 2021/12/14 12:02:08 ...y/files/temp_repo.go:257:CommitTreeWithDate() [E] Unable to commit-tree in temporary repo: user2/initial-unsigned (integrations/gitea-integration-mysql/data/integrations/gitea-integration-mysql/tmp/local-repo/upload.git1503302879) Error: exit status 1
            	Stdout: 
            	Stderr: error: gpg failed to sign the data

        testlogger.go:78: 2021/12/14 12:02:08 .../api/v1/repo/file.go:387:handleCreateOrUpdateFileError() [E] UpdateFile: Unable to commit-tree in temporary repo: user2/initial-unsigned Error: exit status 1
            	Stdout: 
            	Stderr: error: gpg failed to sign the data

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@9d943bf). Click here to learn what that means.
The diff coverage is 51.78%.

@@           Coverage Diff           @@
##             main   #17979   +/-   ##
=======================================
  Coverage        ?   45.28%           
=======================================
  Files           ?      820           
  Lines           ?    90930           
  Branches        ?        0           
=======================================
  Hits            ?    41174           
  Misses          ?    43195           
  Partials        ?     6561           

Impacted Files	Coverage Δ
modules/context/repo.go	52.88% <0.00%> (ø)
modules/templates/base.go	44.89% <ø> (ø)
routers/api/v1/user/gpg_key.go	44.31% <0.00%> (ø)
routers/install/install.go	0.56% <0.00%> (ø)
routers/private/mail.go	0.00% <0.00%> (ø)
routers/private/manager.go	0.00% <0.00%> (ø)
routers/private/manager_unix.go	0.00% <0.00%> (ø)
routers/web/repo/branch.go	56.80% <0.00%> (ø)
routers/web/repo/editor.go	24.15% <0.00%> (ø)
routers/web/repo/pull.go	30.54% <0.00%> (ø)
... and 14 more

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