Improve multiple strings in en-US locale (#26213)

I kept sending pull requests that consisted of one-line changes. It's
time to
settle this once and for all. (Maybe.)

- Explain Gitea behavior and the consequences of each
  setting better, so that the user does not have to consult
  the docs.
- Do not use different spellings of identical terms
  interchangeably, e.g. `e-mail` and `email`.
- Use more conventional terms to describe the same things,
  e.g. `Confirm Password` instead of `Re-Type Password`.
- Introduces additional clarification for Mirror Settings
- Small adjustments in test
- This is a cry for help.
- Grammar and spelling consistencies for en-US locale
  (e.g. cancelled -> canceled)
- Introduce tooltip improvements.

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Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>