Add allowCustomHomeservers config option

[ashhhleyyy]

Description

This adds a configuration option called allowCustomHomeservers, which prevents the Homeserver field on the login and register screens from being edited manually, only from the dropdown menu to the side. This is the same as the disable_custom_urls option for Element.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

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[kfiven]

This also disable homeserver field when someone changes the value.

[ashhhleyyy]

Whoops! That should be fixed now