Use core's "Add Media" language

"Add Content" isn't enough of an improvement to relabel
Change "Insert Content Item" to "Insert Post Element"
Use a different modal title when updating
Check options, which is a bit more semantic
Insert / Update button should reflect context
Change "Return to list view." to "Back to list"
Merge pull request #81 from fusioneng/75-string-tweaking

Tweak modal strings