Validate volunteer entry

[coka]

Any string is currently valid as a volunteer first and last name. For example, it is possible to add a volunteer called "1 .". We should improve this entry form in the following ways:

• Allow only letters.
• Display an error when the user tabs to the next form field if the previous field is invalid.
• Volunteer names should be sanitized before saving them to the database (e.g. leading and trailing whitespace should be trimmed).