[MozillaBugTrackerBridge] Fix incorrect newlines in feed title

[yamanq]

Due to changes in Bugzilla, plaintext returns the bug id with extra whitespace at either end. This can be either by making a narrower (and more brittle) selector or by trimming the plaintext. I chose the latter for this PR.

Test: http://rssbridge/?action=display&bridge=MozillaBugTracker&context=Bug+comments&id=668576&limit=-1&sorting=lf&format=Atom

Old output:

        ...
        <title type="text">
        Bug 668577
       - registerProtocolHandler notification should provide a &amp;quot;don't ask me again&amp;quot; button - Mozilla Bug Tracker</title>
        ...

New output:

        ...
        <title type="text">Bug 668577 - registerProtocolHandler notification should provide a &amp;quot;don't ask me again&amp;quot; button - Mozilla Bug Tracker</title>
        ...