Limit preview deployments for Sanity

[taesungh]

Similar to HackAtUCI/irvinehacks-site#106, it would be good to limit the amount of preview deployments, especially so as we approach the event day and portions of the site are still being worked on. Our team usage dashboard shows we had 48 builds on October 29 and around 25 yesterday, and the limit for Hobby tier is 100 per day. We can theoretically reduce the number of deployments down to half by making preview deployments for Sanity happen only when the pull request includes a corresponding label: see HackAtUCI/irvinehacks-site#107 for implementation.