moves getCycleLength to corresponding files, adds function to get next menses (not finished) with first tests

Resolve "Implement design for day overview"

Closes #134

See merge request bloodyhealth/drip!54

Resolve "extract text from stats and layout"

Closes #129

See merge request bloodyhealth/drip!51

# Conflicts:
#   components/labels.js

Resolve "bug: one-digit hour or minute values can't be saved for temperature"

Closes #126

See merge request bloodyhealth/drip!56

Resolve "add menu bar and basic navigation"

Closes #132

See merge request bloodyhealth/drip!53