Configurable expiry for activation link and password reset link

[bpcurse]

Follow up to nextcloud/server#11857
@Knot3n

If there were individual configuration options for these two expiry times under settings / security, server admins could set the expiry timeout on their own, depending on their environment and needs.

7d could be set as default for the activation link and 1h as default for the password reset link.

[nextcloud-bot]

GitMate.io thinks possibly related issues are nextcloud/server#4008 (Disable reset password link), nextcloud/server#4352 (Password reset button broken), nextcloud/server#7156 (LDAP Invalid private key after password reset), nextcloud/server#7469 (Cannot set password for share by email links), and nextcloud/server#12491 (Fix share link password input).