config: migrate "url_base" dupes to "jfa_url"

URL Base now refers to JUST the subfolder portion, i.e. `/accounts` if
you access jfa-go at `http://jellyf.in/accounts`. General > "jfa_url"/"External
jfa-go URL" now refers to the WHOLE URL you access jfa-go at, i.e.
`http://jellyf.in/accounts`. The settings in "invite emails" and
"password resets" have been removed, and a value chosen from the two
applied to "jfa_url". Migration also makes a config backup. Adds a
"deprecated" flag to config-base, which just tells the UI to not show
it (for now). Also added some warnings related to the URL base /
External URL.

lang/setup/en-us.json

@@ -48,6 +48,8 @@
         "listenAddress": "Listen Address",
         "urlBase": "URL Base",
         "urlBaseNotice": "Only needed if using a reverse proxy on a subfolder (e.g 'jellyf.in/accounts').",
+        "externalURL": "External jfa-go URL",
+        "externalURLNotice": "The URL you'll be accessing jfa-go from. Used to generate links for things like password resets. Make sure to include the above URL base if you set one.",
         "lightTheme": "Light",
         "darkTheme": "Dark",
         "useHTTPS": "Use HTTPS",