Save task ordering in DB

[smaspe]

Use a string index to sort the tasks by alphabetical order

When inserting a task:

• Use the largest of the newly surrounding indices - 1 as new index value. (or the smallest + 1 if there is only a smaller one)
• If the surrounding tasks are immediately following each other (e.g. 'abdgh' and 'abdgi'), use the n-1 and insert an extra letter.
• If the index ends with 'a', insert 'am' instead
• Never let an index end with 'a' (so to keep room above) (there is nothing between 'b' and 'ba', so use 'bam' instead)

Sort by reverse alphabetical order to easily make room for new tasks at the top