Fix markdown image with link

[LER0ever]

This PR fixes the image with link rendering. And therefore resolves #4569 and resolves #5207

Original Behavior

Images with ![alt](imgurl) will be put inside a link to itself, unless it's an svg-file.

The problem is that it hardcoded the svg filetype to be directly rendered in <img> tag, and other file types are rendered like <a><img></a> which causes #4569 and #5207 .

New Behavior in this PR

Images with ![alt](imgurl) will be put inside a link to itself, unless it's already inside a link, which is [![alt](imgurl)](url).
So we don't need to infer from filetype whether the image is a CI image or not.
Also, this matches the Github-Flavored Markdown's behavior on images with links.

[techknowlogick]

Can you add the Gitea copyright at the top of the file?

[LER0ever]

Can you add the Gitea copyright at the top of the file?

@techknowlogick , what do you mean? This is not a new file, and the copyright is already there markdown.go

[techknowlogick]

// Copyright 2018 The Gitea Authors. All rights reserved.

Needs to be added under the Gogs copyright

[LER0ever]

I see, fixed in fdd8ceb .

[codecov-io]

Codecov Report

Merging #4675 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4675      +/-   ##
==========================================
+ Coverage   37.47%   37.49%   +0.02%     
==========================================
  Files         310      310              
  Lines       45921    45922       +1     
==========================================
+ Hits        17207    17217      +10     
+ Misses      26239    26231       -8     
+ Partials     2475     2474       -1

Impacted Files	Coverage Δ
modules/markup/markdown/markdown.go	73.14% <100%> (+6.79%)	⬆️
models/repo_list.go	64.55% <0%> (+1.26%)	⬆️

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[techknowlogick]

@LER0ever Thanks 😄

[lafriks]

Tests would be needed for this

[LER0ever]

@lafriks Tests added for images with links.

Meanwhile, although this implementation is definitely gonna work, I still find this character locating approach a little bit dirty.
So if anyone has a better one that also processes images with links the same way GitHub does, feel free to replace this one.

[lunny]

@LER0ever @lafriks This should be backport to v1.6