Collective-contract-management

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

- Nothing changed yet.

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

- only process IContract objects not IEvent objects
[MrTango]

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

- Add uid and url fields to ICS export.
[MrTango]

- Add summery_prefix to exported event summeries, based on most upper parent Contracts obj.
[MrTango]

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

- Add contracts_ics_view, calendar_from_collection adapter, locales and optimize styling on contract_view

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

- Add contract_amount field to Contract and add an index for it
[MrTango]

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

- Allow nested Contracts containers
[MrTango]

- Make start/end date not required
[MrTango]

- Fix upgrade steps to not show up in addon control panel
[MrTango]