feat(permalinks): point Share links/QR at to.healthchat.ch

Add a config-driven matrix_to_url option so Share dialog links and QR
codes resolve against a matrix.to-compatible resolver instead of the
hardcoded public matrix.to. MatrixToPermalinkConstructor now reads the
base URL lazily from config (default https://matrix.to), keeping the
matrix.to URL format. HealthChat configs point it at the in-house fork
https://to.healthchat.ch and drop the conflicting permalink_prefix
(which routes to the Element-format constructor a matrix.to fork can't
parse).

docs/config.md

@@ -125,6 +125,10 @@ complete re-branding/private labeling, a more personalised experience can be ach
 3. `brand`: Optional name for the app. Defaults to `Element`. This is used throughout the application in various strings/locations.
 4. `permalink_prefix`: An optional URL pointing to an Element Web deployment. For example, `https://app.element.io`. This will
    change all permalinks (via the "Share" menus) to point at the Element Web deployment rather than `matrix.to`.
+   4a. `matrix_to_url`: An optional base URL of a matrix.to-compatible resolver to use for Share links and QR codes instead of
+   the public `https://matrix.to`. Unlike `permalink_prefix` (which points at an Element Web deployment and uses Element's
+   `/#/room/…` URL format), this keeps the matrix.to URL format (`/#/<entity>`), so it must point at a matrix.to deployment or
+   fork. For example, `https://to.example.com`.
 5. `desktop_builds`: Optional. Where the desktop builds for the application are, if available. This is explained in more detail
    down below.
 6. `mobile_builds`: Optional. Like `desktop_builds`, except for the mobile apps. Also described in more detail down below.