Add a read from dumps-only mode

[Standard8]

For development, the search team method is to change the dumps locally, and then clear all data & restart.

This is generally fine, but runs the risk of the collections being updated from the server at some stage after the restart. To avoid this, we think it would be useful if the devtools could put the client into a dump-only mode which would enforce only loading from the dumps.

We could probably set the server url like we do for the in-tree tests.

[leplatrem]

This could potentially require changes on the client implementation, since we only load the dumps when the server URL is the prod server or when running tests (source). That condition was necessary since the data on the dev and stage servers do not match the dumps.

For the reference, this is the value we set:
user_pref("services.settings.server", "data:,#remote-settings-dummy/v1");

[leplatrem]

The typical scenario would be modifying the dump, testing it, modify again & restart to pick up the changes.
So I think there's two parts to this: 1) stopping the sync happening, 2) somehow clearing the database on each restart.
2) might be better if we also update the last modified (would we need to update the build's last modified as well?), though we'd probably need to find a way to script that in editors, so that it would happen automatically.