Add canonical meta tag to view issue page

It is possible for issue URLs to have additional parameters in some
situations. Search engines such as Google will index better if they
know which are the set of canonical URLs. It reduces load through
over-indexing, allows the search engine to prioritise the indexing of
URLs that are correct, and avoids extraneous or non-ideal URLs getting
into the index.

Fixes 0031833, PR https://github.com/mantisbt/mantisbt/pull/1862