Using str_shuffle means that each character can only occur once in the generated string. This reduces the number of permutations possible. For example, using base 36, and 4 characters, there are 36^4 = 1.68M, but the current function can only generate 36*35*34*33 = 1.41M of these. This alternative will use the whole range of possible strings.

Allow character to appear more than once

* Rename plugin and its functions
* Update README with new name
* Update license with current year
* Make plugin refuse to activate if the old 'Normal plugin' version
  is still active.

This was formerly an external plugin named `random-keywords`.

History from the `random-keywords` repo is kept.

If strlen($possible) is for example 36, rand could also return 36.
But substr starts with position 0. So we have to subtract 1 from
strlen($possible).

----

Yoinked from YOURLS/random-keywords#10 by @dgw

The conflict preventer function was broken anyway (ineffective logic AND
incorrect syntax)