Rework filename randomizer character set #1

The current set of characters used in the live version of Imeji includes characters that conflicts within other managed environment setups such as Grav CMS.

The conflicting characters are - and +, and also potentially $. The original implementation for this character set is so that it is easily typeable by humans manually if so needed. However when using urls containing these characters in managed environments they start to break.

Potential Solutions and More Problems:

Removing these characters from the randomizer will solve the problem for future uploaded files but may cause the probability of ID collisions to increase due to having less characters to work with.
Another possible solution is to also increase the number of characters in the filename. This will mitigate the above mentioned problem but also cause the URL to become longer.

More solutions are needed to take on this problem.

SYSTEM_MANUAL_IMPORT
USER:YandolsZX

This has been completed. Merged to master branch and deployed on live environment. Only - and + was removed from the character pool, $ was left alone because it doesn't seem to be that much of a problem. Will reopen issue in case it does.