Recognise URLs like:
   - moz-extension://c66906b4-3785-4a60-97bc-094a6366017e/pages/options.html

Fixes #2657.

This stops |Exclusions| from holding a reference to the |value|
parameter passed to |Settings.set|. In Firefox, this object is garbage
collected when the owning context (the exclusions popup) is closed.

The fix for all such cases in the future is to switch to using
|Settings.get|, which implicitly does |JSON.parse JSON.stringify value|
and thus returns an object in the same context as |Settings|.

We could fix this generally by doing this for the
|Settings.performPostUpdateHook| call in |Settings.set| instead.
However, I'm not convinced that it warrants the overhead of a
|JSON.parse| for every |Settings.set| call.

1. Explicitly remove comments.
2. A trailing backslash means the current line continues on the next line.

E.g.

    a \
      b \
      c

is a single line `a b c`.

This helps alleviate the fact that configuration lines can be very long,
whereas the *Custom key mappings* input is quite narrow.

TODO: We should use the same line parser in the custom-search-engines
input.

Fixes #2299.

Also, separate this regexp out into its constituent parts, because it is
becoming too different to read.

Enable silent/patch releases.