Add single point of entry support

This pull request adds the fabled “single point of entry” checkbox.

Now that we’re using remits to associate councils with courts (#221), we can also use them to identify which courts are a single point of entry for their associated councils. These changes add a `single_point_of_entry` column to the `remits` table, which is exposed through a checkbox in the admin UI and an additional `single_point_of_entry` key in the exported JSON.

This implementation makes a big assumption:

**If a court is a single point of entry for a particular area of law, then it’s the single point of entry for all councils it covers.**

So, for example, if Bristol Civil and Family Justice Centre covers both Bristol City Council and North Somerset Council for any cases involving Children, it can be the single point of entry for *both councils*, or it can be the single point of entry for *neither council*; we are unable to make it the Children single point of entry for Bristol City but not for North Somerset.

My understanding is that this matches the business rules around single point of entry courts.