Fix or remove empty <a> tags in description

From https://gitlab.com/fdroid/repomaker/merge_requests/44#note_30243482

OK, it wasn't the truncate which broke the layout. It is a <a> tag inside the description which causes Django/MDL/whoknows to to bad stuff. Firefox marks the second <a> with the following text:

An "a" start tag seen but an element of the same type was already open.

With this description

<p>Orweb is a companion browser to <a>Orbot</a>, the port of Tor to Android.</p><p>Orbot anonymizes internet traffic by routing it through many different stages and you must have that enabled first, though root isn't needed. Orweb disables certain other browser features that could be used to identify you.</p>

you get the following HTML:

		<p>Orweb is a companion browser to </a><a>Orbot</a>, the port of Tor to Android.</p>
			<p>Orbot anonymizes internet traffic by routing it through many different stages and ...</p>

When manually removing the <a> in the description

<p>Orweb is a companion browser to Orbot, the port of Tor to Android.</p><p>Orbot anonymizes internet traffic by routing it through many different stages and you must have that enabled first, though root isn't needed. Orweb disables certain other browser features that could be used to identify you.</p>

I get a HTML as excepted:

<p>Orweb is a companion browser to Orbot, the port of Tor to Android.</p><p>Orbot anonymizes internet traffic by routing it through many different stages and you mus...

I have tested it with and without JavaScript, it does not change the behavior. So this time it's not #31 (closed) :P

As a fix, I suggest to remove <a> tags from the description when sanitizing.