Merge branch 'illustration-revert' into 'master'

briar-android/artwork/ic_check_circle_dark.svg

briar-android/artwork/ic_check_circle_light.svg

briar-android/artwork/ic_error.svg

briar-android/artwork/ic_exclamation_mark.svg

briar-android/artwork/il_crash_dark.svg

briar-android/artwork/il_crash_light.svg

...ct/briar/android/contact/add/remote/NicknameFragment.java

...java/org/briarproject/briar/android/login/LoginUtils.java

...org/briarproject/briar/android/mailbox/ErrorFragment.java

...g/briarproject/briar/android/mailbox/MailboxActivity.java

...rproject/briar/android/mailbox/MailboxStatusFragment.java

...arproject/briar/android/removabledrive/ErrorFragment.java

briar-android/src/main/res/drawable/ic_check_circle.xml

briar-android/src/main/res/drawable/ic_crash.xml

briar-android/src/main/res/drawable/ic_error.xml

briar-android/src/main/res/drawable/ic_exclamation_mark.xml

briar-android/src/main/res/drawable/il_crash.xml

briar-android/src/main/res/layout/fragment_crash.xml

briar-android/src/main/res/layout/fragment_error.xml

briar-android/src/main/res/layout/fragment_final.xml