Add a list of selectable api targets

[sjefen6]

The client can have a targets.json file with urls of known/popular api targets. The ui should present this with the previously used target or the first in the list as default. At the bottom of the list should there be an "Other" option that enables a text input box for using user specified urls.

[
   "https://api.castcloud.org",
   "http://localhost:8080/api"
]