Add find-as-you-type on the choose template page

Not everyone knows how to use `ctrl` + `f`, and it’s not scoped to
just the list of templates.

The template you want to work with is often not the first one in the
list, but ordering by created at is useful for other reasons (mainly
around first time use).

This commit adds a find as you type control which aims to give users a
quick way of getting to the template they want to work with.

app/main/forms.py

@@ -22,7 +22,7 @@ from wtforms import (
     FieldList,
     DateField,
     SelectField)
-from wtforms.fields.html5 import EmailField, TelField
+from wtforms.fields.html5 import EmailField, TelField, SearchField
 from wtforms.validators import (DataRequired, Email, Length, Regexp, Optional)
 
 from app.main.validators import (Blacklist, CsvFileValidator, ValidGovEmail, NoCommasInPlaceHolders, OnlyGSMCharacters)
@@ -591,3 +591,8 @@ class ChooseTemplateType(Form):
             ('sms', 'Text message'),
             ('letter', 'Letter') if include_letters else None
         ])
+
+
+class SearchTemplatesForm(Form):
+
+    search = SearchField('Search by name')