modify navdrawer header to mailbox

mailbox-android/artwork/navigation_drawer_header.svg

...ndroid/src/main/res/drawable/navigation_drawer_header.xml

mailbox-android/src/main/res/values-ast/strings.xml

mailbox-android/src/main/res/values-bg/strings.xml

mailbox-android/src/main/res/values-br/strings.xml

mailbox-android/src/main/res/values-ca/strings.xml

mailbox-android/src/main/res/values-cs/strings.xml

mailbox-android/src/main/res/values-de/strings.xml

mailbox-android/src/main/res/values-es/strings.xml

mailbox-android/src/main/res/values-eu/strings.xml

mailbox-android/src/main/res/values-fa/strings.xml

mailbox-android/src/main/res/values-fi/strings.xml

mailbox-android/src/main/res/values-fr/strings.xml

mailbox-android/src/main/res/values-gl/strings.xml

mailbox-android/src/main/res/values-he/strings.xml

mailbox-android/src/main/res/values-hi/strings.xml

mailbox-android/src/main/res/values-it/strings.xml

mailbox-android/src/main/res/values-ja/strings.xml

mailbox-android/src/main/res/values-ms/strings.xml

mailbox-android/src/main/res/values-nb/strings.xml