Allow end users to specify servers by URL

[ghost]

It is too inefficient for the server operator to send a pull request for each new LibreSpeed server. There are several Librespeed servers in my country, but none of them are listed in the app.

[vdbhb59]

Provide for them to be added to the server list. Also @adolfintel there should be an option to have the list of public instances and people able to add them via a MR. That makes sense to a FOSS.