HB2-93 make prediction for activity types too

[drixselecta]

Provide realistic schedule estimations for unlogged users: fixes #93

• Only propose activity types in forms for which the athlete has a prediction model
• Calculate realistic defaults for all activity types with activities
• Cleanup existing activities, delete the ones that don't have corresponding Strava activities
• The only way to create a new activity type is by importing a corresponding Strava activity

[coveralls]

Pull Request Test Coverage Report for Build 849

• 147 of 150 (98.0%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 98.927%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
homebytwo/routes/models/activity.py	83	84	98.81%
homebytwo/routes/fields.py	4	6	66.67%

Totals
Change from base Build 838:	-0.05%
Covered Lines:	2306
Relevant Lines:	2331

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💛 - Coveralls