Added an activity indicator for deleting offline copies

[hosy]

Description

Give the user a feedback, when offline copies will be deleted

Related Issue

#393

Motivation and Context

Better user feedback

How Has This Been Tested?

Account > Manage > Delete Offline Copies
Activity Indicator should appear right beside "Delete Offline Copies"

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #438 into milestone/1.1 will increase coverage by <.01%.
The diff coverage is 0%.

@@                Coverage Diff                @@
##           milestone/1.1     #438      +/-   ##
=================================================
+ Coverage          27.22%   27.22%   +<.01%     
=================================================
  Files                264      264              
  Lines              20031    19960      -71     
=================================================
- Hits                5453     5435      -18     
+ Misses             14578    14525      -53

Impacted Files	Coverage Δ
...wnCloud/Bookmarks/BookmarkInfoViewController.swift	0% <0%> (ø)	⬆️
ownCloud/Bookmarks/BookmarkViewController.swift	80.99% <0%> (+1.96%)	⬆️
ownCloud/Client/ClientRootViewController.swift	68.09% <0%> (+2.77%)	⬆️
...ud/Server List/ServerListTableViewController.swift	70.83% <0%> (+4.34%)	⬆️

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[jesmrec]

Approved