Added custom layout for donation popup

app/src/custom/res/drawable/donation_img.xml

app/src/main/java/se/leap/bitmaskclient/EipFragment.java

.../leap/bitmaskclient/fragments/DonationReminderDialog.java

...in/java/se/leap/bitmaskclient/utils/PreferenceHelper.java

app/src/main/res/drawable/cust_button_primary.xml

app/src/main/res/drawable/cust_button_secondary.xml

app/src/main/res/drawable/donation_img.xml

app/src/main/res/layout/donation_reminder_dialog.xml