Activity heatmap: support different days as start of week

[ykahveci]

The activity heatmap on the profile page shows 7 rows for the days in a week, starting on a Sunday:

This might feel natural for many users, however large parts of the world (for example most of Europe) instead use Monday as the first day of the week.
This already covers most countries, though countries also use Saturday as the first day of the week.

The Week article on Wikipedia has a graphic showing which countries use which day as first day of the week, as well as the following explanation:

Cultures vary in which days of the week are designated the first and the last, though virtually all have Saturday, Sunday or Monday as the first day. The Geneva-based ISO standards organization uses Monday as the first day of the week in its ISO week system through the international ISO 8601 standard. Most of Europe and China consider Monday the first day of the (work) week, while North America, Israel, South Asia, and many Catholic and Protestant countries, consider Sunday the first day of the week. [...]
— Wikipedia (Week)

For this reason, I propose that the first day of the week is chosen based on either the browser locale or optionally user preference.
For example, it could look something like this for users that are "used to" weeks starting on a Monday:

[fehmer]

closed with #6385