Introduce fetching client configuration from a remote JSON file

[oskarhane]

Intro

This introduces the ability for a client to be configured by passing an URL to the :config command, like: :config https://oskarhane-dropshare-eu.s3-eu-central-1.amazonaws.com/conf-GHrEFakccy/conf.json

If successfully found config params, the :config frame shows up with the new config in it (just as before).
If any error occurs, an error banner shows up below the editor (new feature introduced with styling support).

To discuss

Two questions that comes up are:

1. Should we restrict fetching hosts to only those in the whitelist? The :style command has no restriction on this.
2. Should we replace the current config, or merge as we do in the current state of this PR? We replace the :style when fetching a new one. We also replace all params when :params {x:2}.

@akollegger @pe4cey ^^

Error banners

User input error i.e. :config x::

Error in remote json file format:

Error in remote server headers:

Example files that can be used:

(first, add oskarhane-dropshare-eu.s3-eu-central-1.amazonaws.com to your server config browser.remote_content_hostname_whitelist)

Correct formatted file with only new config params:
https://oskarhane-dropshare-eu.s3-eu-central-1.amazonaws.com/conf-eKM9GgLJCo/conf.json

Correct formatted file with existing config params in it:
https://oskarhane-dropshare-eu.s3-eu-central-1.amazonaws.com/conf-GHrEFakccy/conf.json

Invalid formatted file:
https://oskarhane-dropshare-eu.s3-eu-central-1.amazonaws.com/conf2-SX8pcCviMC/conf2.json

Non allowed URL (due to their CORS headers):
http://google.com

[oskarhane]

Decisions made on above points to discuss:

1. Yes, restrict remote urls to hostnames in the server config browser.remote_content_hostname_whitelist. Also add this restriction to the :style command.
2. There should be a replace of config, but merged with the default initial config state. That means if you have set maxFrames to 1 and you fetch a remote config that does not include the config maxFrames you should expect it to reset to it's default value. Because that's what it does with the :style and :params commands.

[oskarhane]

Updated and ready for review