Imio.history

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- In `utils.getLastAction`, do not break if history is empty, added tests.
[gbastien]

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- Define a `BaseImioHistoryAdapter` to base new history adapter on.
`checkMayView=True` is now a default parameter of `getHistory`.
[gbastien]
- For now, specifically restrict histories displayed in the `historyview` to
`workflow` and `revision`.
[gbastien]
- Added method `utils.getLastAction` that returns the metadata of last action of
a given name for a given history.
[gbastien]

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- Adapted History word highlighting ti display it bigger and underline it so
it is even more viewable.
[gbastien]

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- In ImioWfHistoryAdapter.historyLastEventHasComments, call
ImioWfHistoryAdapter.getHistory with parameter 'for_last_event=True' so
getHistory knows that it queries only relevant last event and when overrided,
the package overriding it may avoid heavy processing if relevant.
[gbastien]
- Get rid of unittest2.
[gbastien]

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- Do not break in IHContentHistoryView.getTransitionTitle if transitionName
contains special characters.
[gbastien]

----------------

- Make sure comments is displayed correctly by using portal_transforms to
turn it to 'text/html' before displaying it in the PageTemplate using
'structure'.
[gbastien]