Make the composer and TopicCreator use category id instead of category name

Also fixes #1171
Added backwards compatibility for topic creation
FIX: Index out of range error

Fixes 'index out of range error' that occurs when viewing a user's
profile page when they haven't yet posted a reply or created a topic
and is @mentioned in a topic.
proper content-disposition header when downloading attachments
Merge pull request #1417 from ZogStriP/content-disposition

proper content-disposition header when downloading attachments
Merge pull request #1402 from LaunchAcademy/fix_index_out_of_range_error

FIX: Index out of range error
Merge pull request #1215 from Elegios/composer_use_category_id

Make the composer and TopicCreator use category id instead of category name
Fix IE9 bug in browser-update. Unable to get value of the property 'insertBefore': object is null or undefined
Remove brackets from `username_filters[]` URL
improve approval copy
FIX: Don't onebox links with labels unless the labels are the same as the URL
Update importer_spec for system user
Show post number in the share dialog
Revert "Merge pull request #1215 from Elegios/composer_use_category_id"

This reverts commit 763859d4396f406352c4901502a8c33430e54cb2, reversing
changes made to b62caf80fbb81ea7a8ece5632513fcc8559db0e1.
Merge branch 'master' of github.com:discourse/discourse