* Make distinction between DisplayName and Username in email templates

Store the actual username in the variable named Username and store the
separate DisplayName in another variable. This allows us to access the
actual username when we need, which currently fails if a user has set a
full name.

Fixes #6161

* Use u.Name directly

No need for extra function, also change use in all mail sending
functions here

* Don't include Username when not used

* xxx_active_code_live setting in printed in hours and minutes instead of just hours

* Update app.ini description of xxx_code_lives settings

* Added rel="noopener" to target="_blank" hrefs

* Replaced gogs.io/docs with docs.gitea.io

This commit improves templates readability, since all of them use consistent
indent with all template command blocks indented too.

1. Indents both HTML containers such as <div>, <p> and Go HTML template blocks
   such as {{if}} {{with}}

2. Cleans all trailing white-space

3. Adds trailing last line-break to each file

- #1496: fallback plain text
- #1002: add date header
- #913: fix encoding of header