tt-rss: Allow selecting a domain

Currently, the SELF_URL_PATH is being set to http://localhost/tt-rss/ . There are some functions of tt-rss that won't work because of this. For example, opening the rss item with 'o' keyboard shortcut. Also likely there is a feature for getting items as RSS feed that won't work. So, fix this, allow the user to select a domain name in the app interface (similar to Quassel) and set SELF_URL_PATH using this. Also set the protocol to 'https' instead of 'http'.

See: https://discuss.freedombox.org/t/solved-tiny-tiny-rss-opening-link-via-hotkey-o-not-working/1761