Unify search

[martin-ueding]

So I've built a search function in #70 and table of all tracks with filtering options in #174. Then in #212 I've built a time filter for the heatmap. The heatmap also has a filter for activity kinds.

Tools like Jira have their own query language, JQL. It would be nice to have some type of general filter mechanism such that one could filter various things. Also it would allow to unify other things:

• The day overview page is just an overview of the activities of a single day.
• The name overview is the same, just for a different set of activities.

So basically have a search functionality that is a bit more general. And then offer different views of the same data. As a line plot (perhaps limited in number), table, share picture, heatmap.