Change U2F app.ini.sample

[jolheiser]

Creating this issue to tie in any relevant other issues caused by this.
This is the cause of #4260 #4692 #5394 #5406 #5526 (maybe others as well, linking them here for a quick reference to close any as needed with PR, and to link back to this issue)

These lines in the app.ini.sample are referencing a key from a different section (which is invalid), and so when users copy the app.ini.sample over their own, the default values are causing Gitea to crash on startup.

I'd be happy to put in a PR to fix it, however I'd like opinions on the "best" way to do it.

1. We could move those three values out of the [server] section and into the default section so everything has access to them.
2. We could add those three values to the [U2F] section (I don't particularly like this implementation, as it means more redundancy)
3. We could change the sample values to some other generic format and/or comment them out, implying users will need to change them to get them working. i.e. APP_ID = http://localhost:3000/

[techknowlogick]

I like option 3 (commenting out existing values, and adding a note that users need to change them to get them working.