Add high-level integration tests for most important features

[ericfreese]

• Taking a one-time shift (in progress testing take one shift log view #81)
• Taking a recurring shift
• Scheduling an absence (in progress [WIP] - Feature absences spec #82)
• Seeing upcoming shifts on the home page
• Seeing todo pickup reports on the home page
• Seeing shifts that need covering on the home page (Add feature test for viewing shifts needing cov. #112)
• Filling out a pickup report