Convert characters to ascii before cleaning up file names

[jboulen]

We have installed your plugin in my institution and it works great. Thank you for sharing this useful plugin.

We have just added a minor patch.

Currently, when you enable the clean filenames option, the characters with diacritics are removed.

E.g., a file named élèves groupe 5.zip becomes lves_groupe_5.zip

Our patch replaces accented letters with their unaccented equivalents.

So, with the example above, the file name becomes eleves_groupe_5.zip

If you are not interested in this change, you can close this PR. No problem. :)

[gergelyrakoczi]

Dear @jboulen, thank you for this improvement. This makes sense for me; however, first before merging, I will check your change internally.
Cheers,
Gergely

[gergelyrakoczi]

We have checked your contribution and we like it, and will include the code in the next stable version (probably for Moodle 5.1)
Thank you again! @jboulen