feat: Increase attachment size options

[tshuli]

Problem

Closes #662

Solution

• Expand options for attachment size up to 20mb in intervals of 1mb
• Limit options for email mode forms to 7mb

Before & After Screenshots

BEFORE:

AFTER:
(email mode)

(storage mode)

Tests

(tested on IE)

• Create storage mode form. Check that you can set max attachment size up to 20mb. Publish form. Upload file of ~15mb. Check that upload succeeds and you can retrieve it from admin data tab.
• Check that it is not possible to have total attachments across multiple fields exceeding 20mb.
• Create email mode form. Check that you can only set max attachment size up to 7mb. Upload file of ~6mb. Check that upload succeeds and you can receive it in your inbox.
• Check that it is not possible to have total attachments across multiple fields exceeding 7mb
  .

[karrui]

not a fan of being so explicit in the allowed array, hardcoding 1 to 20, but I concede that backwards compatibility probably trumps my opinion.

lgtm

[liangyuanruo]

We probably want to reword these to stop repeating examples - doesn't make much sense UI-wise.

[liangyuanruo]

Is an enum still the best representation, if we allow every integer in the range? Should we model this differently?