Imio-prettylink

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- Do not break in `PrettyLinkAdapter.getLink_cachekey` if context does not have
a workflow.
[gbastien]

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- Added caching for `PrettyLinkAdapter.getLink`, the cachekey returns context,
modified, is_locked, review_state and every parameters defined in
`PrettyLinkAdapter.__init__`.
[gbastien]
- Moved link computation from `PrettyLinkAdapter.getLink` that is now a cached
method to `PrettyLinkAdapter._getLink` so it is possible to call it directly
without caching or to override it.
[gbastien]
- Finalized testing configuration to be able to test the getLink caching.
[gbastien]

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- Added submethod _get_url that does the url computation.
Additionally it manages the fact that context is a Dexterity file and
append relevant part to the url (/download/...).
[gbastien]
- Do not break if icon name contains special characters.
[gbastien]

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- Added CSS class 'no_access' to <span> "can_not_access_this_element"
in addition to class 'discreet' so it may be customized if necessary.
[gbastien]
- Initialize the 'title' attribute with contentValue, this way if a
content is cropped to be displayed (maxLength=...), the complete content
is displayed on hover.
[gbastien]

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- Make sure quotes used in title are not breaking formatted strings,
we escape it by replacing quotes by it's HTML entity &39;
[gbastien]

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- If 'isViewable' is True (default), check that current user have
'View' on the linked element, if it is forced to False, leave it False.
This way, 'View' check to linked element is managed by imio.prettylink.
[gbastien]