Support filtering by multiple selected values

[prusswan]

Currently the only way to filter by multiple selection (using IN operator) is to use enums, so I made a quick hack to get it to work with general integer columns. I can expand this further if there is more interest.

[mshibuya]

Motivation is unclear, why you need this feature and how to use?

[prusswan]

@mshibuya This is to allow filtering using multiple values similar to build_statement_for_enum. Right now if multiple values are passed in for the same integer column, the filtering will work like "AND" (a == 8 AND a == 7). If the column is enum, then it works like "OR" (a == 8 OR a == 7)

[mshibuya]

Change in ra.filter-box.js and main_helper.rb looks unrelated to the topic of PR, please separate them if you still need this feature.