Fragment for MainActivity?

Since this activity is quite sparse we should just have it as a fragment pushed by the scheduling activity (or vice versa, I don't have all of the designs so I'm not sure which would make more sense)--creating activities takes a lot of resources so we should avoid having many of them.

For the sake of merging in all pending changes before the semester's end, feel free to make an issue for this and not change it now.

_Originally posted by @jmf373 in https://github.com/cuappdev/pear-android/pull/3_

Commencez par examiner MainActivity et l’activité de planification, puis comparez-les aux designs d’application disponibles. L’issue n’identifie pas quel composant devrait devenir un fragment et ne définit pas de condition d’achèvement concrète ; clarifiez donc la navigation et le design prévus avant de modifier le code.

Rédigé par le modèle d'indexation à partir du texte de l'issue.