Inconsistent tabbing between text editor and Add Media-button in New Post view

[gish]

Behavior:

1) When the marker is placed in the editor field and the user tabs backwards (SHIFT+TAB), the selected element getting focus is the input field for page slug. Some buttons are skipped ("Add media", "Change permalinks")

2) Tabbing forward from the input field, the skipped buttons are focused.

Expected behavior:

1) The select phase tabbing between editor field and "preview" button should be: Editor field <->"Add media"<->"Change permalinks" <-> Slug input field

[SergeyBiryukov]

Related: #22933

[adamsilverstein]

the attached patch corrects the tab order on the new post page; added an id to the Add Media button so it can be targeted in the tab order;

also corrects related #22933

[markjaquith]

In 23395:

Fix some tabbing issues on the post editing screen. Add Media no longer skipped when going backwards from content. Save Draft no longer skipped when going forwards from content.

props adamsilverstein. fixes #23195 and #22933