Rename branding forms to clarify who they're for

I've often struggled to find the form associated with a particular
page due to the overlapping names e.g. "SetEmailBranding" sounds
more like the radio button form a user sees than "BrandingOptions".

Almost every form in forms.py also ends with "Form", so this also
makes the branding forms consistent with that naming convention.

app/main/views/letter_branding.py

@@ -12,8 +12,8 @@ from notifications_python_client.errors import HTTPError
 from app import letter_branding_client
 from app.main import main
 from app.main.forms import (
+    AdminEditLetterBrandingForm,
     SearchByNameForm,
-    ServiceLetterBrandingDetails,
     SVGFileUpload,
 )
 from app.s3_client.s3_logo_client import (
@@ -48,7 +48,7 @@ def update_letter_branding(branding_id, logo=None):
     letter_branding = letter_branding_client.get_letter_branding(branding_id)
 
     file_upload_form = SVGFileUpload()
-    letter_branding_details_form = ServiceLetterBrandingDetails(
+    letter_branding_details_form = AdminEditLetterBrandingForm(
         name=letter_branding['name'],
     )
 
@@ -123,7 +123,7 @@ def update_letter_branding(branding_id, logo=None):
 @user_is_platform_admin
 def create_letter_branding(logo=None):
     file_upload_form = SVGFileUpload()
-    letter_branding_details_form = ServiceLetterBrandingDetails()
+    letter_branding_details_form = AdminEditLetterBrandingForm()
 
     file_upload_form_submitted = file_upload_form.file.data
     details_form_submitted = request.form.get('operation') == 'branding-details'