Tablet drag handling (rebased onto dev_5_0)


This is the same as gh-2256 but rebased onto dev_5_0.

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Fixed a couple of issues from https://trello.com/c/YIKlaBpm/37-tablet-issues-rfes-with-web-client-main-window.

To test, first find a tablet (iPad and Android if possible). NB: we should probably buy an Android for the office!

- Check that the whole webclient fits into the window and there's no loss of the right edge of the right panel.
- Try to resize the main left and right panels of the webclient by dragging the handles (still a little tricky but possible).