Issue #1798880 by andyceo, LinL, mtunay: Convert teaser_length() to use configuration system.

PR for https://github.com/backdrop/backdrop-issues/issues/170
Hey @hellomrcat! While this code works perfectly, it's not exactly what I meant. If the function definition is updated to be:
```
function text_summary($text, $format = NULL, $size = 600) {
```

Then 600 is used if nothing is passed into the $size parameter. Then we can delete this entire IF statement. Same result, less code. :)