Sticky rich text controls

### As a content editor editing long pages, I want the rich block text controls to remain sticky at the top of the page so that I don't have to keep scrolling up and down to format content.

**Is your feature request related to a problem? Please describe.**
I input a lot of mobile vendor food guide content into Joplin staging; these are long pages where the text often goes below the fold. I had to keep scrolling above the fold to access controls like `H2`, etc.

**Describe the solution you'd like**
The rich text block controls to remain sticky at the top of the screen until I've scrolled past that text box,

**Describe alternatives you've considered**
*A clear and concise description of any alternative solutions or features you've considered.*

**Additional context**
*Add any other context or screenshots about the feature request here.*

Start by locating the rich block text controls in the editor and reproduce the issue with a long page like the mobile vendor food guide content described. Implement and manually verify that the controls stay at the top while their text box is in view, then stop when the page has scrolled past that text box.

Written by the indexing model from the issue text.