dev/core#942 fix failure to render names for some activities

dev/core#942 fix failure to render names for some activities

Overview
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Set limit for activity_contact retrieval to 0, allowing to retrieve more than 25 activity contacts when rendering the first 25 activities on the activity contact tab

Before
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![before](https://user-images.githubusercontent.com/336308/57439801-e42a0580-729a-11e9-80a1-45df93d0c5eb.jpg)

After
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![after](https://user-images.githubusercontent.com/336308/57439960-39fead80-729b-11e9-9701-acd79ff73497.jpg)

Technical Details
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This moves the logic for retrieving the target contacts back into the getActivities function. We are stil not wanting to bypass the ACLs so still using the
api but strictly limiting the number of contacts we retrieve (at the cost of extra queries, but cheap ones).

Some tests added on the Bulk Mail activity.

Comments
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