Better logic for handling of filenames with # in them when generating links in markdown

[mrsdizzie]

Right now there are a few bugs trying to turn relative links with a # in them into valid links using our various parsers. This is a combination of assuming anything that starts with a # is an anchor in goldmark.go and not escaping the filename in util.URLJoin.

Some test cases to fix for reference:

https://try.gitea.io/mrsdizzie/testcase/src/branch/master/%231readme.md
https://try.gitea.io/mrsdizzie/testcase/src/branch/master/my%231readme.md

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