searchAndActivate: rework 'activity_count' parameter

- It's not deprecated anymore.
- When activities are grouped at CMFActivity level, it specifies a number of
  activities that are generated at a time instead of a number of activity
  groups.
- The special None value means no limit.
13 jobs for for_testrunner_3 in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:04:26

passed ERP5.CodingStyleTest-TestRunner3

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failed ERP5.UnitTest-Master

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failed ERP5.UnitTest-TestRunner3

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failed ERP5.UnitTest-Zope2

02:05:57

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:06

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:40

passed Wendelin.UnitTest-Master

00:46:54

passed ERP5.CodingStyleTest-Master

00:57:48

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:38

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:40

passed SlapOS.Eggs.UnitTest-Master.Python3

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failed Wendelin.UnitTest-Master

01:03:57