Remove foreign id from form and entry migrations

[ryanmitchell]

Fixes: #97

[what-the-diff[bot]]

• Removed foreign key constraint from entries table
• Added primary key to string id migration file for entries table
• Changed form_id column in submissions table to unsignedBigInteger and removed cascadeOnDelete() method call

[ryanmitchell]

@FrittenKeeZ Why don’t open a PR with the changes you want and ill close this one.

[FrittenKeeZ]

@ryanmitchell if we just skip the orphan handling of submissions, it's a few tweaks only by slapping ->index() on the fields in question and preserving the primary id thing for uuid entries

Edit: I'll do a PR shortly with the index changes