If the login is taking long - add a warning about potentially conflicting add-ons

[tasn]

Related to #237

Suggested in https://www.reddit.com/r/EteSync/comments/rpwsnv/comment/hqtcjki/?utm_source=reddit&utm_medium=web2x&context=3

[at-cf]

Disabling both fingerprint protection setting in "Privacy and Security" and uBlock Origin allowed login.