Fix --port setting (#13288)

* Fix --port setting

Unfortunately there was an error in #13195 which set the --port
option before the settings were read. This PR fixes this by
moving applying this option to after the the settings are read

However, on looking further into this code I believe that the setPort
code was slightly odd.

Firstly, it may make sense to run the install page on a different
temporary port to the full system and this should be possible with
a --install-port option.

Secondy, if the --port option is provided we should apply it to both
otherwise there will be unusual behaviour on graceful restart

Thirdly, the documentation for --port says that the setting is
temporary - it should therefore not save its result to the configuration

(This however, does mean that authorized_keys and internal links may
not be correct. - I think we need to discuss this option further.)

Fix #13277

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Update cmd/web.go

* Apply suggestions from code review

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

docs/content/doc/usage/command-line.en-us.md

@@ -40,6 +40,7 @@ Starts the server:
 
 - Options:
     - `--port number`, `-p number`: Port number. Optional. (default: 3000). Overrides configuration file.
+    - `--install-port number`: Port number to run the install page on. Optional. (default: 3000). Overrides configuration file.
     - `--pid path`, `-P path`: Pidfile path. Optional.
 - Examples:
     - `gitea web`