Misc bugfixes

[niccofyren]

☝️ Sorry for the short description

Feel free to add hotfixes to this PR/branch and we can merge once things seem to have settled

List of patches:

• Renames "next" button on last application step to "submit" - Relates to: https://github.com/gathering/wannabe/issues/48
• Fixes broken redirects after email step during user creation (Replaces POST redirects with a "continue" link) - https://gathering.slack.com/archives/CG3MSB329/p1573300308078400
• Misc Auth issues
• Add missing PHP modules to docker image used for translations
• Make sure correct cache is emptied when application questions are updated (to make sure changes are visible)