* Make clicking "Unknown" language do nothing.

#97

* Implement user cache using embedded H2.

* Separate caching to its own service

#110

* Change UserProfile to be not Serializable

* Fix copy-paste errors.

* Replace Jackson with Gson which reduced boilerplate code by a bit.

* Introduce an extension function

* Remove unnecessary JsonDeserializer class.

Instead, I decided to use a simple TypeAdapter in order to deserialize number into java.util.Date.

* Switch back to Jackson as deserializer.

* Add warning suppression.

* Replace redundant creation of ObjectMapper.

* Replace LocalDateTime with Instant.

* Prevent multiple invocations of generateUserProfile.

* Cache by lazy.

* Remove unnecessary JSON deserialization from lookUpInCache.

* Prevent NPE.

* Update src/main/kotlin/app/CacheService.kt

Co-authored-by: Sidd <iProdigy@users.noreply.github.com>

* Revert "Prevent multiple invocations of generateUserProfile."

This reverts commit 8306d92

* Remove unnecessary custom deserializer.

* Rename function.

UserService#lookUpInCache → selectJsonFromDb

* Reduce invocations of CacheService#selectJsonFromDb.

* Extract functions from duplicate code.

* Update src/main/kotlin/app/CacheService.kt

Co-authored-by: Sidd <iProdigy@users.noreply.github.com>

* Remove unnecessary class UserNotLoadableException.

* Remove unnecessary lazy initialization.

* Change id values to lowercase in DB.

follow-up change to 80154a0

* Change data type of `timestamp` from 'TIMESTAMP WITH TIME ZONE' to 'TIMESTAMP'.

* Use HikariCP.

* Update src/main/kotlin/app/util/HikariCpDataSource.kt

Co-authored-by: Sidd <iProdigy@users.noreply.github.com>

* Replace string concatenation with multi-line string

* Replace uses of magic numbers

* Remove intermediate local variable

* Make CacheService have `HikariCpDataSource.connection` as its field

Co-authored-by: Sidd <iProdigy@users.noreply.github.com>