members_table.vue - refactor dynamically called methods

Summary

Follow-up to !57633 (comment 541093138)

showActionsField and tdClassActions are dynamically called which is fairly confusing for engineers that are looking at the code for the first time. If we remove this dynamic calling we could potentially improve discoverability.

Risks

Low risk, these components are well tested

Involved components

• members_table.vue
• app/assets/javascripts/members/constants.js