CRM-19626: Add min-amount handling to priceset

[jitendrapurohit]

• CRM-19626: Event Registration page allows registration even if no number (or the number 0) has been entered in any ticket quantity boxes

[monishdeb]

UI Review

1. Created price-set that support minimum amount 0
   

2. two Price fields created, one with text numeric and second select field with price-option $0

3. On Event registration page selected $0 price option
   

4. Was able to register successfully.
   

Also checked with all kind of price fields, it works fine. Checked with non-zero minimum amount and validation is respected well if user submit amount lesser then allotted minimum amount.

[yashodha]

merging as it is working as per comments

[colemanw]

@jitendrapurohit @yashodha Putting schema changes directly into the sql file means that an upgrade will fail if run twice (e.g. in the case of upgrading an RC -> stable). I've added a utility for safely adding columns (and we could add more such utilities for other small tasks like this) and moved this code here: #9548

[jitendrapurohit]

@colemanw Thanks for the PR!