Add support for 8-character TOTP secrets

[burymaddie]

I recently received an 8-character TOTP secret from this Jira 2FA plugin: https://marketplace.atlassian.com/apps/1220048/2fa-for-jira-u2f-totp?hosting=datacenter&tab=overview

Trying to save it into an entry results in a "Please match the requested format." pop-up. However, I was able to submit it by adding 8 "A"s to the end of it (i.e. filling remaining hex bits with 0s). This resulted in the proper codes being generated. Would be great to have native support for this.

i.e.
Original seed that cannot be saved: AZM057MJ
Seed that was accepted: AZM057MJAAAAAAAA