adapt to last-minute instant-onboarding changes

[r10s]

deltachat/deltachat-android#3060, deltachat/deltachat-android#3064, deltachat/deltachat-android#3068 are the outcome of one-to-one discussions during the current gathering; see there fore reasoning and discussion.

in summary, the following changes are needed in onboarding:

• pull translations
• create a regular secondary button with the string Use Other Server (instant_onboarding_show_more_instances) below "Agree & Create Profile"
• on tapping Use Other Server, an action sheet with the following options should open:
  • List Chatmail Servers (instant_onboarding_other_server) - do the same as "Explore Other Options" before
  • Classic E-Mail Login (manual_account_setup_option) - do the same as "I Already Have A Login / Manual Login" before
  • Scan Invitation Code (scan_invitation_code) - do the same a "Scan QR Code" before
• remove the lower two buttons "Explore Other Options" and "Scan QR Code"
• remove "I Already Have A Login / Manual Login" (options left are "Add as Second Device" and "Restore From Backup") (from WelcomeViewController)

the following changes are needed in "New Chat":

• rename "Scan QR Code" to New Contact (menu_new_contact)
• rename "New Contact" to Add Contact Manually (menu_new_classic_contact); and hide it when isChatmail() is true

the following changes are needed in "Scan QR" view controller:

• add Add Contact Manually (menu_new_classic_contact) to the three-dot-menu and open the old manual contact adding activity

in manual contact adding activity:

• use the title Add Contact Manually (menu_new_classic_contact)
• add the hint new_classic_contact_explain below the input fields