Additional validation and escaping for file names.

This adds additional front and backend validations for task order file
names. We are now restricting file names to a whitelist regex of
[A-Za-z0-9\-_ \.] for simplicity.

Note:
On the frontend, the filename string must have at least one character.
This is not true in the backend validation; because of the way the
entire task order form is validated, requiring input would break the
business logic currently implemented.

templates/components/upload_input.html

@@ -49,6 +49,9 @@
       <template v-if="sizeError">
         <span class="usa-input__message">{{ "forms.task_order.size_error" | translate }}</span>
       </template>
+      <template v-if="filenameError">
+        <span class="usa-input__message">{{ "forms.task_order.filename_error" | translate }}</span>
+      </template>
       {% for error, error_messages in field.errors.items() %}
         <span class="usa-input__message">{{error_messages[0]}}</span>
       {% endfor %}