Support specific deadline for each category of project.

As per title

app/forms/project.py

@@ -8,15 +8,26 @@ from wtforms import (
     TextAreaField,
     validators,
 )
-from wtforms.validators import DataRequired, ValidationError
+from wtforms.validators import DataRequired, StopValidation, ValidationError
+
+
+def empty_to_none(form, field):
+    if field.data == "":
+        field.data = None
+    else:
+        raise StopValidation()
 
 
 class ProjectForm(FlaskForm):
     title = StringField("Title", validators=[DataRequired()])
     description = TextAreaField("Description", validators=[DataRequired()])
-    background_skills = TextAreaField("Background skills")
+    background_skills = TextAreaField(
+        "Background skills", validators=[empty_to_none, validators.Optional()]
+    )
     is_student_proposal = BooleanField("Student Proposal", default=False)
-    on_behalf = StringField("from", default=None)
+    on_behalf = StringField(
+        "from", default=None, validators=[empty_to_none, validators.Optional()]
+    )
     category = SelectField("Category", coerce=str)
     meeting_modes = [("", "N/A"), ("remote", "Remote"), ("in-person", "In person")]
     meeting_mode = SelectField(