Fix bug in wiki link rewriter filter

What does this MR do?

There is a bug in the wiki link filter when the link is relative and it points to the url. Instead of avoiding prepeding the wiki base path we do it twice resulting in links like: /foo/bar/-/wikis//foo/bar/-/wikis/uploads/72bda26c98be192882344365b1796e00/file.png

In this MR we fix that by detecting if the wiki base path is already present.

Steps to reproduce

The bug can be easily reproduced using Gollum Tags. These are the steps:

• Edit a wiki page
• Upload an image attachment. A resulting route like this uploads/530525687890586e4ac0996315bdfd0a/file.png will be added
• Then add the following: * [[uploads/530525687890586e4ac0996315bdfd0a/file.png]] (using the attachment relative path)

The image won't be displayed and if we inspect the url we will see something like:

Where we can see that the absolute path is repeated twice.

Disclaimer: After this fix the image won't be displayed because there is another bug that I'm fixing in !56638 (merged) but if we inspect the element, the url should be right.

Does this MR meet the acceptance criteria?

Conformity

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