Auto-fill matching credentials by default

[antelle]

Investigate the possibility of auto-filling credentials without a user action when there's only one matching credential. So that when you load a page, the username and password is inserted.

This may be too complicated and bug-prone. There's no commitment or guarantee that it will be ever implemented, this issue is to track thoughts around it.

[antelle]

Originally reported as #5

[whysthatso]

although i've used that functionality on the old KeePassHttp-Connector (and it worked quite nicely, showing a dropdown below the fiedl if there where multiple options), i'd rather not have this convenience as deep integrations into js-heavy sites might become hacky and indeed error prone. that simple click on the extension icon to do that is fully sufficient, i think. let's keep this simple.