Fix further issues with missing file attachments.

The previous fix for missing file attachments broke once the `f.close()`
call was moved into the `finally:` block if the file attachment was
missing, due to the fact that the initial statement failed and `f` was
not a valid variable. This change fixes this by creating two separate
`try/except` blocks.

It also sandboxes the generation of text-based file attachment comment
thumbnails, which would also break the page if the file was missing.

Reviewed at https://reviews.reviewboard.org/r/6444/
Prevent various inline editors from vertically shifting on open/close.

The inline editors in various places (replies to reviews/comments and in
the new review dialog) would shift a bit vertically when opening or
closing the editor. This was due to the pencil disappearing and the line
height shrinking where it used to be.

We had this solved for banners. Now, that solution is contained in a
macro, which is used in the other cases where we display these.

Reviewed at https://reviews.reviewboard.org/r/6446/
Remove dead code from `diffReviewableview`

Remove some unused variables and computations from `DiffReviewableView`'s
`_updateCollapseButtonPos` method.

Reviewed at https://reviews.reviewboard.org/r/6455/