Handled storage of absolute image urls

[rshbhgrg]

refs #10024

• Updated input serializers for posts/tags/users to handle absolute urls conversion

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1. Ghost stores images without domain
2. API V2 returns images with absolute urls
3. Ghost-Admin sends absolute urls back on any save e.g. update user

Current behavior: This will override the relative image path in db to absolute, which in turn won't get updated in future if domain or protocol changes for e.g.
Fix: On save/update, input serializers converts any absolute image url paths back to relative if the base URL from image fields matches the configured URL

[kirrg001]

@gargol Could you pls review and do a functional test (including subdirectory)? I'll do a final review asap 👍 Thank you :)

[naz]

Found an issue during testing, image url is not properly handled when moving blog to a subdir: "url": "http://ghost.local", -> "url": "http://ghost.local/blog/". In the latter example, the image is still being served from: /content/images/2018/10/DltA_pyXgAA-Lw_.jpg-large.jpeg. Probably should be tackled with this PR, but maybe would make sense to break things out a bit if it's critical to release a partial fix to just a domain change issue 🤔 ?

[rshbhgrg]

Discussed #10025 (comment) : Its not caused by changes here, there are known problems in changing domains/blog url right now since content is not re-written, so will not be patched in this PR.

[kevinansfield]

problems in changing domains/blog url right now since content is not re-written

To clarify, changing subdomains is a manual process because we don't know which stored URLs are meant to be relative ghost root and which are meant to be relative to the domain root. We'll open separate issues and PRs to handle that side of things if it's something we want to try automating in the future.