Remove placeholder from search

User research has showed us that users were seeing the placeholder text and assuming they could only put in strings of those types. A specific example; users needed to search by device manufacturer in the search field, but didn't because the placeholder was 'eg drug or device name'

This commit removes the placeholder handling for keyword field.
Merge pull request #123 from alphagov/remove-placeholder-text

Remove placeholder from search
Move date suggestion text out of placeholder

Multiple changes here:
- The date filters are now way more flexible than they were, so I've changed the suggestions to show two accepted date formats as examples.
- Use "for example" instead of eg because in previous filters, users seemed to treat eg as ie, meaning they would only use the search box with the suggested format types. Since we can fit "for example" in this placeholder text we should use it instead.
- To avoid replication of the same text, and because the date fields are already visually linked by the grey box, have one helptext at the bottom of the box, rather than one for each input.