Demo search on frontend

### What's done:
* Replaced stub demoList with functional one
* Added demo filtering
* Implemented useDebouncedDeferredRequest

(#2057)

Author: sanyavertolet

save-demo/src/main/kotlin/com/saveourtool/save/demo/controller/DemoController.kt

@@ -19,6 +19,7 @@ import org.springframework.web.bind.annotation.RequestParam
 import org.springframework.web.bind.annotation.RestController
 import reactor.core.publisher.Flux
 import reactor.core.publisher.Mono
+import reactor.kotlin.core.publisher.toMono
 
 import reactor.kotlin.core.util.function.component1
 import reactor.kotlin.core.util.function.component2
@@ -43,14 +44,15 @@ class DemoController(
      */
     @PostMapping("/demo-list")
     fun getFilteredDemoList(
-        @RequestBody(required = false) filter: DemoFilter = DemoFilter.any,
+        @RequestBody(required = false) filter: DemoFilter?,
         @RequestParam(required = false, defaultValue = DemoDto.DEFAULT_FETCH_NUMBER.toString())
         demoAmount: Int = DemoDto.DEFAULT_FETCH_NUMBER,
-    ): Flux<DemoDto> = blockingToFlux {
-        demoService.getFiltered(filter, demoAmount).toList().map { it to demoService.getStatus(it).block() }
-    }
-        .filter { (_, status) -> status in filter.statuses }
-        .map { it.first.toDto() }
+    ): Flux<DemoDto> = filter.toMono()
+        .switchIfEmpty(DemoFilter.any.toMono())
+        .flatMapMany { demoFilter ->
+            blockingToFlux { demoService.getFiltered(demoFilter, demoAmount) }
+        }
+        .map { it.toDto() }
 
     /**
      * @param organizationName saveourtool organization name

save-demo/src/main/kotlin/com/saveourtool/save/demo/service/DemoService.kt

@@ -14,7 +14,6 @@ import com.saveourtool.save.utils.StringResponse
 import com.saveourtool.save.utils.blockingToMono
 import com.saveourtool.save.utils.switchIfEmptyToNotFound
 
-import org.springframework.data.domain.Page
 import org.springframework.data.domain.PageRequest
 import org.springframework.stereotype.Service
 import org.springframework.transaction.annotation.Transactional
@@ -95,7 +94,7 @@ class DemoService(
      * @param pageSize amount of [Demo]s that should be fetched
      * @return list of [Demo]s that match [DemoFilter]
      */
-    fun getFiltered(demoFilter: DemoFilter, pageSize: Int): Page<Demo> = demoRepository.findAll({ root, _, cb ->
+    fun getFiltered(demoFilter: DemoFilter, pageSize: Int): List<Demo> = demoRepository.findAll({ root, _, cb ->
         with(demoFilter) {
             val organizationNamePredicate = if (organizationName.isBlank()) {
                 cb.and()
@@ -114,6 +113,8 @@ class DemoService(
             )
         }
     }, PageRequest.ofSize(pageSize))
+        .filter { demoFilter.statuses.isEmpty() || getStatus(it).block() in demoFilter.statuses }
+        .toList()
 
     /**
      * @param demo [Demo] entity
@@ -154,6 +155,7 @@ class DemoService(
      * @param projectName saveourtool project name
      * @return [Demo] connected with project [organizationName]/[projectName] or null if not present
      */
+    @Transactional(readOnly = true)
     fun findBySaveourtoolProject(
         organizationName: String,
         projectName: String,