Extract MessagesPage data loading to dataAccess module

[richardolsson]

This PR moves the data loading required for MessagesPage to the dataAccess module as a first step towards implementing cache revaliation. See issue #25 for game plan and motivations.

This is joint work by me and @WULCAN. We realized that further extraction and refactoring is possible, but chose not to do it now as it is not crucial for the cache issue, and can be done at a later point.

[WULCAN]

I know I was part of writing this but I feel pretty confident in my ability to review it well anyway.

I approve of merging these changes but leave it up to you to merge it, just in case you want a more independent review first.