Fix section editing bug

[snan]

Editing sections often remove the last character of the previous
section. We can't have that.

The workaround has been to try to remember to always add an extra
newline manually at the start of each section we're editing but that
gets tedious and doesn't fix all issues.

[snan]

As a demonstration of the issues this commit addresses, make a page like this:

# test för section buggen

## fifi

## rhrh

## gugu

Foo bar

### eheh

### nini

and mess around with the edit section buttons, you don't even have to add any text, just edit a section and save it and then look at the full page source in between and do that a couple of times.

New lines before section starts gets eaten and

Foo bar

becomes

Foo ba
r