Allow custom URL permalinks (configure custom matrix.to instance)

### Your use case

#### What would you like to do?

I would like element to make it easier to use other `matrix.to` instances like `mto.vern.cc`

Currently, the only way to do that is to modify the `matrix-react-sdk` and replace all instances of `matrix.to` with the instance.

I would like a way where in the config, I can set all the matrix.to URLs that are recognized by element.

#### Why would you like to do it?

To help distribute the load/usage of matrix.to

#### How would you like to achieve it?

an option for the recognized matrix.to URLs in `config.json `

### Have you considered any alternatives?

For a while, I tried to subs_filter `matrix.to` with `mto.vern.cc` but the hack caused many glitches and weird behaviours.