Save context when merging topics.

Closes #2166
Merge pull request #2189 from wordpress-mobile/issues/2166-save-when-merging-topics

Save context when merging topics.
Make back button title a single space.

Fixes #1045. Without the space physical devices display the standard "Back" label.
Convert tabs to spaces.
Clear cached row heights when switching topics.
Omit protocol when searching for image URL.
Removing the media spinner titleView after media download is complete.
Merge pull request #2197 from wordpress-mobile/issues/2187-doubled-images

Omit protocol when searching for image URL.
Merge pull request #2194 from wordpress-mobile/issues/1045-cropped-back-button

Make back button title a single space.
Fix check for max posts.
Use correct margin width when calculating max label width.
Do not photonize a private blog's images.
Merge pull request #2206 from wordpress-mobile/issues/2205-do-not-photon-private-images

Do not photonize a private blog's images.
Merge pull request #2203 from wordpress-mobile/issues/2192-crash-max-posts

Fix check for max posts.
Bumping version to 4.3.
Merge branch 'issue/media-spinner' into release/4.3
Fix for NSURLErrorDomain error -999 on post preview.
Merge pull request #2211 from wordpress-mobile/issues/2162-preview-fails-sporadically

Fix for NSURLErrorDomain error -999 on post preview.
Fixed state of spam button.
New approve/unapprove icons.