This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* Works with the current column layout exported as Excel file
* Tries to fix invalid CSV when it wasn't exported from Excel
* Imports categories
* Imports topics into the correct category
* Allows skipping archived topics
* Allows skipping private topics
* Makes use of the latest features from the base importer
* Some minor fixes and documentation updates
