Cache settings #4125

[benjaoming]

Builds upon #4122

Fixes one main issue:

• File based caching is not persistent because it's stored in a tempdir!

New feature:

• The dev settings now include memory based caching by default
• Docs mention need to clear webcache at one of the settings where it's necessary
• Settings for caching much less messy
• More efficient topic_tree view now that it doesn't have to write the topic tree to a useless file-based cache
  Default cache settings are weird #4125

[benjaoming]

@MCGallaspy assigning you because it's an easy task IMO... just check that the code doesn't have any obvious problems... it's been tested in 3 alphas now without issues.

[rtibbles]

Seems OK to me, and has been tested.