Make MATHESAR_CLIENT_DEV_URL configurable via .env

[dmos62]

Allows setting MATHESAR_CLIENT_DEV_URL in the .env file. I sometimes need this when working out of a remote box.

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[silentninja]

Approving as it is needed for development workflow and doesn't break anything!

[dmos62]

What's happening with the merge queue? You'd expect this PR to have no effect on any check.