Provide a settings servlet for loklak_webclient

[Orbiter]

Main purpose for such a servlet is, that application keys must be shared. We want to host such key in loklak_server and loklak_webclient can use the same keys too.

It shall work like this:

• the loklak_webclient reads settings from their custom_configFile.json
• it then reads a json from /api/settings.json. The json contains properties in the same format as custom_configFile.json. Every entity thats inside those properties from the server overwrites the settings as read from custom_configFile.json

[Orbiter]

Implemented with https://github.com/loklak/loklak_server/commit/9d12a22d6f79b1df82a6d2fb4c6b57b88d7393b8

To propagate settings to the /api/settings.json servlet, set those settings within the conf/config.properties file with a prefix 'client.'. That key prefix will be truncated when settings are transmitted within the servelet.