SAK-49199 Mailsender: Check for exceeding attachment size Client Side

[mylescarey2019]

Adds client side attachment file size testing vs sakai.properties mail.sendfromsakai.maxsize limit (or the default value of 25MB). If sum of attachments exceeds the limit a warning banner is presented instructing user to remove one or more file until below the limit. Along with warning banner, control to add more files is disabled.

[mylescarey2019]

@ern - fyi - latest suggestions have been implemented