Element-web: Electron should support pulling custom config.json from preferences if available

[ara4n]

You can't edit the config.json in-place in Riot on Electron on some platforms (e.g. macOS) as it breaks the package signature.

Instead, we should support looking at ~/.riot/config.json or equivalent to let users override the default config.json (e.g. to provide a custom installation script which grabs Riot and then applies the custom config.json to set up the correct default HS etc).

[t3chguy]

Related/would solve https://github.com/vector-im/riot-web/issues/4394#issue-237851822

[ara4n]

actually this is p1 to support folks who want custom desktop apps. @t3chguy do you think you'll have a chance to look at it? otherwise we'll put it on the team proj board.

[t3chguy]

I will have this in a PR tomorrow

[ara4n]

thanks! :)