Remove the lines that checked for existing upload with the same source.
These lines were breaking replacements in many cases, so we had to
always empty the post's source before replacing, and they served no
purpose since the md5 check already catches duplicate uploads.
When doing a tag search, we have to be careful about which user we're
running the search as because the results depend on the current user.
Specifically, things like private favorites, private favorite groups,
post votes, saved searches, and flagger names depend on the user's
permissions, and whether non-safe or deleted posts are filtered out
depend on whether the user has safe mode on or the hide deleted posts
setting enabled.
* Refactor internal searches to explicitly state whether they're
running as the system user (DanbooruBot) or as the current user.
* Explicitly pass in the current user to PostQueryBuilder instead of
implicitly relying on the CurrentUser global.
* Get rid of CurrentUser.admin_mode? (used to ignore the hide deleted
post setting) and CurrentUser.without_safe_mode (used to ignore safe
mode).
* Change the /counts/posts.json endpoint to ignore safe mode and the
hide deleted posts settings when counting posts.
* Fix searches not correctly overriding the hide deleted posts setting
when multiple status: metatags were used (e.g. `status:banned status:active`)
* Fix fast_count not respecting the hide deleted posts setting when the
status:banned metatag was used.
* Add MediaFile abstraction. A MediaFile represents an image or video file.
* Move filetype detection and dimension parsing code from uploads to MediaFile.
* Rename Upload#download_for_upload to #get_file_for_upload.
* Fix #get_file_for_upload to raise error if no file or source url was given.
* Fix javascript upload validation to disallow submitting form if file is
not present and the source is not an url.
