Wallets should dynamically obtain the list of selectable lightwalletd servers

[nuttycom]

Proposal: Originally mentioned in #15 (comment)

• The wallet's current server is always stored in the format of the custom field. It can be set at the same time as the initial migration, but then should only be manually changed via the server selection screen from then on.
• The set of options to change to is provided from a CDN-hosted file, plus the custom option.
• If the currently selected option matches one of the options downloaded from the file, then that option should be the one marked selected, otherwise it is displayed in the custom field.
• If no options are available from the CDN-hosted file, then just the custom field is shown.
• The data format chosen for providing this information should also include space for a "blacklist" of servers, so that if a previously-promoted lightwalletd instance is discovered to be compromised, wallets can be forced to disconnect from the blacklisted server and prompted to choose an alternative. Users should have the option to explicitly override the blacklist designation, and should also be provided with adequate information to decide whether they need to take further action beyond just switching servers.

[daira]

TODO(@daira): post the security requirements any such mechanism must satisfy.